First year blues. We pinched today September 13 Autumn Reds, Freedoms, and Winter Rose. Most of our plants are growing real well inspite of our struggle to keep the house cool through our 90 degree weather.

I am struggleing with understanding initiation. What kind of conditions will make plants initiate? what will stop it? How do we know that it happening?

Our sunrise now is 7:24 Am and Sunset is 8:04 PM. How short of day will initiate flowers?

I just realized that there are 6 outside lights here at the school. How do we know if they are bright enough to delay initiation? Ant If we have to tarp the plants at night how long is adequate?

Any help would be appreciated.

Victor Noble

The rule of thumb that I use is that if you can see the veins in the leaves while in the greenhouse at night it is too bright. We have a couple of lights in the adjacent portables near our greenhouse that shine in, but they have never been a problem. In fact we usually come in a little early with all of our varieties. I will begin next week posting pictures of each of our varieties next week on our website (http://www.lanierffa.org).

Hi Victor

This might help....

I'm a graduate student at the University of Florida studying initiation in poinsettia, so I'm dealing with questions like those you've posted in my research.

First off when thinking about night length the data you should consider is 'civil twilight' not sunrise/sunset. You are right that poinsettias are senstive to low light levels, so at sunrise and sunset they will be sensing daylight. Civil twilight gives a better estimation of perceived night length. Tables are avaiable online from the US naval observatory at http://aa.usno.navy.mil/.

How short of days will initiate flowers will depend on the cultivar and the temperature of your greenhouse. In high temperatures, a longer night length will be needed. In cool conditions, early cultivars like Autumn Red and Freedom will initiate at about 11 hours and 15 minutes night length (civil twilight), but if your average daily temperature is above 80 degrees it will closer to 11,30. Your later cultivar winter rose will need about 11,30 cool or 11,45 warm.

Lights at night will not allow initiation. I agree with Tracey as far as how bright you need to be concerned with. I check by holding my hand over the plant and if I see a shadow on the leaves, the light is too bright. If you find the lights are too bright and turning them off is not an option, begin black clothing when you want initiation. Lights on all night will prevent normal floral development as well, so you will need to continue covering untill they flower.

How do you know flowering is happening? The first sign will be bract color developing several weeks after initiation, followed by apearance of the flower bud in another 1-2 weeks.

Rebecca

Hi Victor -

Poinsettias are short day or long night responsive. It has been determined that the night length needs to be approximately eleven hours 40 minutes for the flower initiation process to take place. In most of the northern hemisphere the natural day length is short enough around September 25th. Some varieties such as Freedom, Autumn Red, and Early Winter Rose initiate early; possible even as early as September 15th. During the immediate days after flower initiation it is impossible to determine whether the plants have initiated with the naked eye. Later a hand lense can be used to see the structures forming. Visible bud is the first time the flower visible without the use of a hand lense. This usually occurs about 30 days after the start of short days.

If short days are being provided to shorten the day length growers usually reduce the daylength to 10 hours. Blackclothing is used to encourage the plants to initiate early or to block out any unwanted light pollution from street lights, security lights, etc. It only takes 10 ft. candles at plant height to inhibit or delay the process. Tracey's rule is a good one to follow if you don't have a light meter to actually measure the levels. You should only be able to make out the silhouette of the plants and not see the veins at night in the greenhouse. If light pollution will be a factor for you we recommend that the plants be black clothed until 1-2 weeks prior to complete flower development to avoid any unwanted delays in the process.

Hope this helps. Please let us know if you have further questions. Good luck!

Thank you very much. I can really sound intelligent to my students when I have so many willing to share advice.

Did I understand correct that I need only black cloth till the flowers form? After black clothing and flowers form is light control important anymore? How many weeks of black clothing is required to form flowers?

Hi Victor -

If light pollution is not a factor than you would only need to black cloth until the natural photoperiod is short enough. For Freedom that will be in the next few days. If you are worried about the light pollution and it is enough to inhibit or delay flower initiation than we recommend blackclothing for at least 7 weeks from the time the blackclothing began. When you are dealing with light pollution the plants will probably still initiate flowers but the development process is slowed and you might not have a saleable crop by Christmas. Black clothing to exclude the pollution ensures that the crop will be ready on time.

Hope this makes sense. Thanks!

Just a passing thought. Rebecca mentioned the hand shadow test to see if the light was too bright. What does a full moon do to poinsettias, it sometime will cast a shadow.

Victor Noble

That's a good question. It has been our experience that while a full moon does seem to create enough light to be termed "pollution" we have never experienced a delay from what we felt was the result of a full moon.